mm/z3fold.c: allow __GFP_HIGHMEM in z3fold_alloc

One of the gfp flags used to show that a page is movable is
__GFP_HIGHMEM.  Currently z3fold_alloc() fails when __GFP_HIGHMEM is
passed.  Now that z3fold pages are movable, we allow __GFP_HIGHMEM.  We
strip the movability related flags from the call to kmem_cache_alloc()
for our slots since it is a kernel allocation.

[akpm@linux-foundation.org: coding-style fixes]
Link: http://lkml.kernel.org/r/20190712222118.108192-1-henryburns@google.com
Signed-off-by: Henry Burns <henryburns@google.com>
Acked-by: Vitaly Wool <vitalywool@gmail.com>
Reviewed-by: Shakeel Butt <shakeelb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>